1. Field of Invention
The present invention relates to a frequency lock filtering receiver and the corresponding method that can be applied to receiving radio frequency signals.
2. Related Art
Superheterodyne is a method that uses local oscillation signals to mix with input signals and converts the frequency of input signals into a specific frequency. Superheterodyne receivers have the advantages of sufficient and stable amplification, high selectivity and ease in adjustment. However, they have some special problems such as images and nearby channel interferences.
With reference to FIG. 1, in a conventional superheterodyne receiver, an antenna 11 receives a radio frequency (RF) signal. A band select filter 12 performs band filtering. A low noise amplifier (LNA) 13 amplifies the RF signal. An image rejection filter 14 rejects images of the RF signal. A frequency synthesizer 17 generates a local oscillation signal needed when a mixer 15 lowers the frequency. The mixer 15 mixes the RF signal and the local oscillation signal to obtain an intermediate frequency signal. The intermediate frequency signal is filtered by a channel select filter 16 to obtain an intermediate frequency signal with nearby channel interference removed. The modulated signal is then amplified by a power amplifier 18, filtered by an emission band select filter 19 and transmitted to the antenna 111 for emission.
In particular, the image rejection filter 14 and the band select filter 12 are discrete components and cost higher. Furthermore, the frequency bandwidth is fixed, therefore it has to include all RF signals to be received. Its abilities to reject images and nearby channel interference should be improved.
In view of the foregoing, the present invention provides a frequency lock filtering method and device for receivers that can accurately filter signals and has strong abilities in rejecting images and nearby channel interferences.
The present specification discloses a frequency lock filtering receiver and the method for the same. The receiver uses a frequency synthesizer to generate the frequency of radio frequency (RF) signals to be received in a tuning mode. The central frequency of a tunable image rejection filter is adjusted through feedback control so that the central frequency locks in the RF to be received. Afterwards, the receiver enters a receiving mode to receive RF signals using an antenna and uses the central frequency of the tunable image rejection filter locked in the tuning mode to filter the RF signals and to receive signals.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.